In a control system of a digital mixer, first class devices are connected via a first class bus, and second class devices are connected to each first class device via a second class bus. The number of first class devices, which require a specific function for adding source and destination addresses to data, can be reduced by the number of the second class devices. Moreover, the second class device which does not require the specific function for adding source and destination addresses to data input/output operations is cheaper than the first class device, resulting in the implementation of simplified configuration and reduced production costs.